Data reduction is required in data handling processes, where too much data is present for practical applications using the data. Generally speaking, digital images--images that have been discretized in both spatial coordinates and in brightness levels such as those acquired by scanning--are often large, and thus make desirable candidates for at least one form of data reduction. These digital images do not typically change very much on a pixel to pixel basis and have what is known as "natural spatial correlation." For example in a digital color image, it is commonly known that the required spatial resolution for chrominance data is less than the spatial resolution for luminance data. The natural spatial correlation enables reducing not only the digital image data, but the resources required by certain image processing operations on the reduced data.
Digital color images may be described in terms of the chrominance and luminance values for each pixel contained therein. It is obviously desired to reproduce color images such that the colors in the copy exactly, or at least closely match the corresponding colors in the original image. Since image input and output devices are often quite different, reproducing an accurate color image often requires some form of color correction to be applied to the chrominance and luminance data before it is output. Color correction in digital images for a printing device is an image processing operation which may include a correction from 3-D device independent color space (e.g., YC.sub.r C.sub.b, RGB, XYZ, or L*a*b) to a 3-D device dependent color space; and then a conversion to CMYK comprising under-color removal (UCR), gray-component replacement (GCR) and linearization processes.
The time required for color conversions such as those described above is directly proportional to the amount of data to which it is applied. Thus, it is desirable in many applications to employ some form of data reduction in order to facilitate rapid image processing. In addition to compression, subsampling schemes are used in scanners, digital copiers or other devices that are used to reproduce, store or process color documents. Briefly, a subsampling scheme involves selecting some subset of the available original image data for subsequent image processing operations. This substantially reduces the volume of data that is subsequently generated and converted, preferably with little or no impact on the appearance of the reproduced image. Any combination of luminance and chrominance data where at least one of the chrominance channels is at a reduced density is generally referred to as subsampled luminance and chrominance data.
There are various luminance-chrominance color spaces, including the CIE standard L*a*b*, and L*u*v*; and industry standard YCrCb. One could also define a simply computed luminance-chrominance space for a specific purpose. The distinguishing feature of a luminance-chrominance space is that one of the three axes represents the luminance, or lightness of the color, while the other two together represent the hue (related to the color name) and colorfulness or purity. For simplicity, we use LCrCb to mean any luminance chrominance space with L as the luminance channel and Cr and Cb as the other two channels, which represent the difference R-G of the amount of red and green, and the difference B-Y of the amount of blue and yellow respectively that is present in the image at a given pixel.
The present invention may be used to reduce the resources required to reproduce a digital color image. The invention may be used when full resolution luminance and chrominance data is available, or when the data has been previously reduced using subsampling, compression or other known techniques. Rather than performing color correction on all of the pixels in an image, the method disclosed selects one pixel in each block for full conversion from the LC.sub.r C.sub.b color space to the CMYK color space. Device values are then assigned to the remaining pixels by combining the converted device data for the selected pixel with the luminance data for the remaining pixels.
The following disclosures may be relevant to aspects of the present invention:
U.S. Pat. No. 4,275,413 to Sakamoto et al. issued Jun. 23, 1981 discloses a color space transformation where information is placed into lookup tables and stored in a memory--where the lookup table relates input color space to output color space. Sakamoto teaches a "unit cube interpolation unit" having known vertices. The lookup table is commonly a three dimensional table since color is typically defined with three variables.
U.S. Pat. No. 5,581,376 to Harrington issued Dec. 3, 1996 teaches the conversion of input device signals Rs, Gs, Bs, generated by an image input terminal, to calorimetric values Rc, Gc, Bc, the colorimetric values being processed to generate address entries into a lookup table to convert them to Cx, Mx, Yx, Kx colorant signals or any multi-dimensional output color space, which includes but is not limited to CMYK or spectral data. Values not directly mapped may be determined using tetrahedral interpolation over a hexagonal lattice where the lattice is formed by offsetting every other row in at least one dimension.
U.S. Pat. No. 5,477,345 to Tse issued Dec. 19, 1995 relates to subsampling processors and a three color sensor array that may be employed to supply subsampled chrominance data to a printing machine, a computer memory device or other device.
U.S. Pat. No. 5,067,010 to Ishii et al. issued Nov. 19, 1991 discloses a color video signal processing device in which pixels are thinned out for a whole picture plane with respect to each of two kinds of digital color difference signals in accordance with a predetermined role. The encoding is executed on a unit basis of a block consisting of (n.times.m) samples where (n and m are integers no less than 2) which are formed with respect to each of the two kinds of color difference signals whose pixels have been thinned out or a block consisting of (n.times.m) samples formed so as to include both of the two kinds of color difference signals whose pixels had been thinned out. The data compression is executed on a block unit basis.
U.S. Pat. No. 4,656,515 to Christopher issued Apr. 7, 1987 discloses a television display including circuitry for reducing the amount of memory needed to hold one field of the reduced size image. In the display apparatus, digital samples representing the large and small picture signals are developed at substantially equal rates by separate circuitry. Subsampling circuitry stores one out of every five of the samples representing a horizontal line of the small picture. These samples are displayed, synchronous with the large picture at a rate three-fifths times the display rate of the large picture samples to produce an apparent size reduction of one-third in the horizontal direction.
Pending U.S. patent application Ser. No. 08/537,056 by deQueiroz et al. titled Fast Preview Processing for JPEG Compressed Images filed Sep. 29, 1995 and assigned to the assignee of the present invention discloses a method of rapidly decompressing a document image compressed using transform coding for scaling and previewing purposes. A fast algorithm is derived by utilizing a fraction of all available transform coefficients representing the image. The method is particularly efficient using the discrete cosine transform which is used in the JPEG ADCT algorithm. In JPEG ADCT, a very fast and efficient implementation is derived for a resolution reduction factor of 16 to 1 (4 to 1 in each direction) without needing any floating point arithmetic operations.
Pending U.S. patent application Ser No. 08/721,130 by deQueiroz titled Method and Apparatus for Processing of a JPEG Compressed Image filed Sep. 26, 1996 and assigned to the assignee of the present invention discloses a method and apparatus for processing images that have been compressed using a discrete cosine transform operation, and particularly JPEG compressed images. In a preferred embodiment, the rotation of image blocks is accomplished by sign inversion and transposition operations to accomplish intrablock operations. Subsequently, one of a number of alternative methods is employed to accomplish the same image processing on an interblock level, thereby enabling the rotation or mirroring of compressed images. The two stage process allows the use of either a standardized JPEG system with enhancements or a hybrid processing method, thereby accomplishing the image processing in conjunction with compression or decompression operations and minimizing the need for large memory buffers to accomplish the image processing. Accordingly, the technique has application to any number of systems, including digital printers and copiers where there may be a necessity to orthogonally rotate or mirror the digital image.
Pending U.S. patent application Ser. No. 08/770,765 by Klassen et al. titled Color Correction of a Compressed Image filed Dec. 19, 1996 and assigned to the assignee of the present invention discloses a method for color correcting digital images that have been compressed. In a preferred embodiment, aspects of the color correction are carried out on the compressed image data to improve computational efficiency. One of a number of alternative methods is employed to accomplish the color correction on lossy or losslessly compressed images. The color transformation process accomplishes color correction on compressed image data in conjunction with compression or decompression operations. A second, simplified phase of the color correction may be applied subsequently to the decompressed image data in certain embodiments. Accordingly, the technique has application to any number of color imaging systems, including digital printers and copiers where there is a necessity to color correct compressed digital images.
Cross reference is made to concurrently filed U.S. patent application also entitled Method and Apparatus for Correcting Luminance and Chrominance Data in Digital Color Images, by Klassen et al., assigned to the assignee of the present invention, and identified as D/97120 discloses a method and apparatus for improving the efficiency of color correcting luminance and chrominance based data. More specifically, the invention is directed to a method of applying a full conversion between color spaces for one pixel in a selected pixel block. Chrominance values are assigned to the remaining pixels based upon their luminance values relative to the luminance value of the converted pixel. The invention may be used to convert between various types of color spaces, and may be used in the processing of both device dependent and device independent data.
All of the references cited herein are incorporated by reference for their teachings.
Accordingly, although known apparatus and processes are suitable for their intended purposes, a need remains for a method of improving the efficiency of color correcting luminance and chrominance based data that will be provided to an image processing system.